Electrostatic reproduction processes are well known to the art. In the well-known Xerox system a photoconductive surface carried by a drum is electrostatically charged by a corona discharge device. The photoconductive surface which may be of selenium or the like is an insulator in the dark and a conductor in the light. The image to be reproduced is focused on the photoconductive surface. When the light strikes the photoconductive surface the charge leaks away in the illuminated areas leaving the dark areas to form the image. The latent image in the form of electrostatic charges on the photoconductive surface is then developed by a toner. This toner must be in the form of a dry powder. The dry toner particles are then transferred by an electrostatic charge to sheet material such as ordinary paper and are usually formed from thermoplastic resins. The toner particles are then fixed by heat on the ordinary paper and the image appears in its final form. The necessity of using heat to fix the toner particles prevents high-speed operation of the Xerox system. Furthermore, the dry toners are slightly abrasive and sooner or later they scratch and mar the selenium surface of the drum which is the heart of the Xerox machine. The dry toner also causes mechanical problems since the toner particles become air-borne and permeate the bearings of the machine. This requires frequent cleaning, which is an onerous task. The dry toners have inherent dielectrophoretic properties which prevent their filling in large black areas. This is easily observed by viewing any copy which has a large, black area made on the Xerox machine.
In the Electrofax method, a sheet of paper is covered with a photoconductor such as zinc oxide. The latent electrostatic image can be developed either by a dry toner or by a liquid in which a toner is suspended. A liquid developer comprises finely divided pigment particles having an average size no larger than about twenty microns and probably much smaller to about an average size of about five microns, suspended in a relatively non-conductive light hydrocarbon such as benzene, zylene, hexane, naphtha, cyclohexane, or the like. The final image, of course, in the Electrofax process, appears on the photoconductor-coated surface and is not transferred to ordinary paper. It has been realized for some time by those skilled in the art that it would be desirable to use liquid developers in a transfer process. No one has found a way, however, to accomplish this conveniently. In U.S. Pat. No. 3,261,688 issued May 17, 1966, to Mihajlov, one attempt at the employment of liquid developers in a transfer process is shown. Mihajlov applies a film of liquid developer over the photoconductive surface. He then exposes the photoconductive surface and the developer simultaneously to a pattern of light and shadow. He then attempts to transfer the image to an ordinary paper. He has found, however, that the non-image areas have background toner. He attempts to reduce this by applying an electrical potential of the same polarity as that of the toner to the roll which presses the paper against the film of developer and makes the roller of conductive rubber or the like.
In all of the processes of the prior art in which transfer of an image is made from a photoconductor to ordinary paper, there is a loss of definition according to the application of pressure which distorts the developed image or through the difficulty of attaining effective transfer of development particles to a new location and the effective fixing of the same. Difficulty is also experienced when the same photoconductive surface is used repetitively, and images produced on it successively are transferred to another medium. The problem is that the photoconductive surface becomes soiled. This soiling is not very great when dry toners are used but in a method such as that of Mihajlov, the liquid toner will adhere to some extent to the photoconductive surface and gradually become unusable.